The present invention relates to distance detection and measurement systems using a radar.
In these systems, the radar periodically emits, for short intervals of time, an electromagnetic wave at an emission frequency situated in an ultra high frequency band. The pulses thus emitted are propagated in the atmosphere, are possibly reflected by a target and are again received by the radar after a length of time which depends on the distance of the radar from the target. The measurement of this time gives a precise indication about the distance of the radar from the target. Moreover, for Doppler radars, the shift between the frequency of the electromagnetic wave of the emitted pulse and the frequency of the electromagnetic wave of the pulse received after reflection from the target gives an indication of the component of the travelling speed of the target relative to the radar.
The pulses are emitted periodically with a recurrence frequency F.sub.r (period T.sub.r). For an observation radar whose antenna rotates while scanning the space to be observed, a piece of data to be taken into consideration is the number N of direct "shots", that is to say the number N of pulses emitted and capable of being reflected by a target before the target leaves the field of the radar because of the rotation thereof. The number N depends on the directivity of the antenna and on the recurrence frequency of the pulses, but it forms a parameter which may be well known for a given system.
If a target sufficiently distant from the radar receives a pulse, the echo reflected can only be received by the antenna of the radar after a second pulse has been emitted, separated from the first one by the recurrence period T.sub.r. In this case, the radar system does not know a priori if the echo received comes from the second pulse reflected by a near-by target or from the first pulse reflected by a much more distant target.
More generally, if a target is at a distance less than cT.sub.r /2 (c being the speed of propagation of the electromagnetic waves emitted), the radar will receive an echo from a pulse during the recurrence period T.sub.r which immediately follows emission of that pulse; it will be said to be a first recurrence or first trace echo.
If the target is at a distance between cT.sub.r /2 and cT.sub.r, the radar will receive an echo during the second recurrence period following emission of the pulse; it will be a second recurrence or second trace echo.
If the target is at a distance between (n-1)cT.sub.r /2 and ncT.sub.r /2, the radar will receive an echo during the n.sup.th period following emission of the pulse; it will be an n.sup.th trace echo, the target being also called n.sup.th trace target.
The pulses are regularly emitted by the radar and it (or PRF) is necessary to know with certainty if an echo received is a reflection of the immediately preceding pulse or not. In order to eliminate ambiguities, different means have then already been proposed for eliminating the echoes which are not first trace echoes.
For example, a variation of the recurrence period from pulse to pulse causes the first trace echoes from a target at a given distance to remain fixed with respect to the pulses which immediately precede them, whereas the other echoes move. The unstable echoes can then be eliminated and only the stable echoes corresponding to first trace targets are kept.
An aim of the present invention is to allow not only first trace echoes to be distinguished from other echoes but also echoes corresponding specifically to the second, third trace, etc . . . to be identified and these echoes to be classified instead of eliminating them so as to determine the exact distance of the targets up to a distance equal to several times the ambiguity distance cT.sub.r /2 of the radar.
Solutions which come naturally to mind for increasing the range as far as possible while still being able to unambiguously determine the distance of targets is would be to multiply by 2, 3, 4 etc . . . the recurrence period or to modulate it in accordance with the principle outlined in the preceding paragraph so as to eliminate echoes other than first trace echoes. But, the first solution reduces the probability of detecting a target whereas the second is ill-adapted to a Doppler radar and is difficult to use.